A number of factors which alter the responsiveness of the renin angiotensin system are being studied. These factors may obscure the true prevalence of low renin hypertension. Age, diabetes mellitus, and drugs that inhibit prostaglandin synthesis depress PRA responsiveness to various stimuli even in the normotensive states. These factors must be carefully controlled before classifying a hypertensive patients as having low renin hypertension. Dopamine-beta-hydroxylase activity was studied and found to be unaltered in hypertensive patient, except in those with severe diabetic neuropathy, and in these patients it was low, as was PRA. The role of the renin system in regulating the blood pressure in normotensive subjects was studied with Saralasin (P113). Only after the stress of sodium depletion and the upright posture could the renin system be shown to play a role in regulating blood pressure in the normotensive state. A group of patients with elevated urinary 17-ketosteroid excretion, oligomenorrhea, hirsutism and hypertension are being studied for excess mineralocorticoid excretion. In a few, episodic increases in plasma Doc have been detected, and two of these patients treated with dexamethasone have become normotensive.